The present invention relates to gas pressure boosters. More specifically, the present relates to gas pressure boosters and methods and systems for using them in which fugitive emissions are captured, and in which the need for staged gas compression is largely eliminated.
Gas pressure boosters may include a drive system which provides the energy required to operate a compression system, and a compression system which elevates the gas pressure. The drive systems may include: a crankcase driven by an electric motor or an engine; a turbine drive; a hydraulic piston driven by an electric motor or an engine; and a pneumatic piston driven by air or gas pressure.
The compression system may include:                a reciprocating piston (providing moderate boost ratios and flowrates, suitability for high operating pressures, low to moderate cost, a compact design, rod seal leakage and vibration, and a moderate operating life for the seals, especially non-lubricated seals);        a turbine (providing high flowrates, low vibration, a long operating life, suitability for high pressures, low boost ratios, high cost, shaft seal leakage, and a large size);        a diaphragm (providing high compression ratios, no seal leakage, suitability for high pressures, very low flow, high cost, vibration, and a low operating life);        a bellows (providing no seal leakage, moderate cost, low flow, low boost ratios, vibration, and a lack of suitability for high pressures);        a rotary vane (providing high flowrates, low cost, low boost ratios, a lack of suitability for high pressures, and a low operating life);        a fan (providing high flowrates, low cost, very low boost ratios, and a lack of suitability for high pressures);        a “roots type” blower (providing high flowrates, moderate cost, long life, low boost ratios, shaft leakage, and a lack of suitability for high pressures); and        a rotary screw (providing high flowrates, moderate cost, long life, moderate boost ratios, shaft leakage, and a lack of suitability for high pressures).        
With moderate to high pressure applications, which are the focus of the preferred embodiment described below, the most practical boost system utilizes a reciprocating piston. Existing piston gas boosters utilize pneumatic or gas drives, crankcase drives and hydraulic drives. For many applications, such as natural gas compression for pipelines, refueling natural gas vehicles and vapor recovery from gas wells, compressed air is either not available or available in sufficient quantity to drive the compression section. Electricity may also not be present, either at all or in sufficient quantities to operate a crankcase or hydraulic drive. There is also frequently a lack of space to locate crankcase-driven machines and engine drives. Accordingly, a compact gas-driven booster may be a good engineering fit in such applications which have high pressure gas (instead of compressed air) available to drive the booster.
Pneumatically-driven gas pressure boosters, also called gas boosters, booster compressors and air amplifiers, that utilize compressed air (or other compressed gases) as the motive force to boost gas pressure, are known. They may be used to boost shop air pressure, to boost nitrogen pressure, for boosting gas pressure to feed dry gas mechanical seals on turbo compressors (to protect the seals), etc. Gas pressure boosters have various advantages: the pressure boost in such devices can be as low as 5 psi or as high as thousands of psi; they require no electricity, cooling water or lubrication; and they are explosion-proof, compact, easy to install and economical. Such advantages may be important in applications located in remote areas where the electricity may not be available (e.g. oil and gas wells). The gas pressure boost ratio is based on the pressure of the available compressed air (or gas), the area ratio between the drive piston and the boost piston, and the boost gas supply pressure. Pneumatically-driven gas pressure boosters are available, for example, from Midwest Pressure Systems of Bensenville, Ill.
With existing pneumatically-driven gas pressure boosters, the boost section includes a single-acting or double-acting cylinder, and inlet and discharge check valves for each pumping chamber. There are variations in check valve design, piston and rod seals, and materials, but all of the existing systems are similar in engineering design.
The pneumatic drive section of the boosters may have several variations, but all consist of a four-way valve which causes the drive piston to reciprocate automatically.
The differences are in the manner used to actuate the valve:
1. Mechanical actuation causes the four-way valve to shift as a result of the drive piston, mechanically moving the valve element at the end of stroke. The piston typically has a lever or pin which triggers the valve at the end of stroke in each direction.
2. Pneumatic pilot shifting actuates the four-way valve through a small amount of pressurized air or gas which forces a piston attached to the valve to move, causing the valve to shift. There are three versions of this design. A first version uses a four-way valve with a double-pilot design which receives a pilot signal at each end of the valve. With this first version, pilot valves are triggered by the piston at the end of each stroke. Each pilot valve sends a pilot air or gas signal to the four-way valve, causing it to shift. After the four-way valve shifts, the pilot air or gas is vented. The second version uses the same two pilot valves, but one valve sends a pilot signal to the pilot side of a single-pilot, spring-return, four-way valve. The pilot air or gas shifts the four-way valve against the spring and remains trapped in the pilot section until the other pilot valve is tripped, venting the air or gas in the pilot section. With this second version, the spring then shifts the four-way valve back to the original position. The third version is similar to the second version. Pilot air or gas actuates a larger pilot piston on one side of the four-way valve and holds it in place. The piston on the other side of the four-way valve is smaller and is always charged with supply air or gas. When pilot air or gas is vented from the first piston the smaller piston shifts the four-way valve back to its original position.
3. Existing booster designs vent the drive air or gas to atmosphere. The pilot air or gas also vents to atmosphere. The drive force is determined by the pressure of the drive air or gas above atmospheric pressure. The flow capability is a function of this drive force as well as the amount of drive air or gas that is available. Typically, the maximum pressure rating of gas booster drive systems is 10 bar or 150 psi, which encompasses the shop air pressure available in most industrial applications.
Rod seal design and materials, piston seal design and materials, and structural materials vary in the pneumatic drive section, but the various models are similar in engineering design.
There is a need for using gas pressure boosters in applications such as oil and gas wells, natural gas pipeline compressor stations, and turbine compressor applications in oil refineries and chemical plants. In many cases, such as gas wells or remote pipeline compressor stations, no electricity is available (all equipment may be run off of natural gas). Available gas pressures can be substantial (e.g., 200-1000 psi). In a gas well, for example, gas from the well enters a separator to remove oil and water. The gas is filtered and transported to a “sales line,” which collects gas and transports it to a natural gas processing facility. Sales line pressure may be in the 100-250 psi range. The wellhead gas at a much higher pressure may be reduced in pressure when it enters the sales line, where substantial energy is lost. The oil separated from the gas is sent to an atmospheric pressure condensate storage tank where gas flashes out of the pressure-reduced stream and continues to bubble out of the oil and water at near-atmospheric pressure (flash gas). This gas is typically vented to atmosphere or burned in a flare. The venting of gas-operated controls at the well also is released to the atmosphere or through the flare. It would be advantageous to develop a system for capturing these fugitive gas emissions from the well and recovering this vented gas and returning it to the sales line. Further, if the energy lost in the reduction of gas pressure were used in the fugitive emission vapor recovery effort, there would be little or no energy cost.
These fugitive emissions of volatile organic compounds are a safety and environmental hazard. In Colorado, environmental standards were put into place in December of 2006 in an effort to reduce volatile organic compound emissions which create ozone and negatively effect air quality. These standards were made more stringent after May of 2008 when condensate tanks emitting more than 20 tons per year of volatile organic compounds are required to reduce emissions by 95% to help reduce the high levels of ozone concentrations in the area and keep Colorado in compliance with national air standards.
Vapor recovery requires boosting the fugitive emissions from near atmospheric pressure to a pressure level where they can be returned to the process. When gas is compressed from a low pressure to a significantly higher pressure, it must typically pass through several stages of compression to remove the heat generated during compression. The additional stages of compression require more equipment and cooling between the stages resulting in additional capital costs and energy consumption. Development of equipment which can reduce the number of stages of compression and utilize the gas pressure potential energy available at the source is very desirable.